Text/Post Background-How to Change?

I’m from Germany and very new to WordPress especially the CSS-Features. For my little german blog about games: ( portableplaying.com ) I added a new background-image. But now you can’t read the text in the articles anymore. Can you tell me how I can change only the text-background to white e.g. using CSS?
It would be great if i could get help….

Note that you should also check other pages like archives and single posts and pages too. For example, the title is class=”archive_header” on archive pages instead of class=”blog-title” like it is on the home page.

Next, you may also want to adjust the spacing of things differently. Here’s a start: